1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode LCD (Liquid Crystal Display) device and, more particularly, to an IPS mode LCD device in which a common electrode and a pixel electrode are formed as a dual layer of a transparent electrode and an opaque electrode, and to the fabrication method of the IPS mode LCD device.
2. Description of the Related Art
Recently, as interests on information displays and demands for using a portable (mobile) information medium are increasing, research on and commercialization of a light thin film type flat panel display (FPD), which can substitute the existing display device CRT, are actively ongoing.
Among the FPDs, in particular, an LCD, a device for expressing an image by using an optical anisotropy of liquid crystal, exhibits excellent resolution, color display and picture quality, so it is actively applied for a notebook, a desktop monitor or the like.
A twisted nematic (TN) mode is one of driving methods generally used for the LCD device, wherein liquid crystal molecules on nematic are driven in a vertical direction to a substrate. The TN mode LCD is, however, disadvantageous in that its viewing angle is some 90°, which is quite narrow. This results from a refractive anisotropy of liquid crystal molecules. That is, when a voltage is applied to the liquid crystal display panel, liquid crystal molecules aligned horizontally to the substrate are aligned nearly vertical to the substrate.
Thus, there has been proposed an IPS mode which improves the viewing angle by more than 170° by driving the liquid crystal molecules horizontally to the substrate, which will now be described in detail.
FIG. 1 is a plan view showing a portion of an array substrate of an IPS mode LCD according to a related art. Generally, in the LCD, a N-number of gate lines and a M-number of data lines cross each to form a N×M number of pixels, but for the sake of explanation, only one pixel is shown in FIG. 1.
As shown in FIG. 1, a gate line 16 and a data line 17 defining a pixel region are arranged vertically and horizontally on a transparent glass substrate 10, and a thin film transistor (TFT) 20 (a switching device) is formed at the crossing of the gate line 16 and the data line 17.
The TFT 20 includes a gate electrode 21 connected to the gate line 16, a source electrode 22 connected to the data line 17 and a drain electrode 23 connected to a pixel electrode line 18L. The TFT 20 also includes an insulation film (not shown) for insulating the gate electrode 21 and the source/drain electrodes 22 and 23, and an active layer (not shown), namely, a channel layer, for forming a conductive channel between the source and drain electrodes 22 and 23 by a gate voltage supplied to the gate electrode 21.
In the pixel region, a common electrode 8 and a pixel electrode 18 for generating horizontal electric field are alternatingly disposed in a longitudinal direction of the data line 17.
At this time, the pixel electrode 18 is electrically connected with the pixel electrode line 18L connected to the drain electrode 23 through a first contact hole 40A, and the common electrode 8 is electrically connected with a common electrode line 8L disposed in parallel to the gate line 16 through a second contact hole 40B.
The common electrode 8 and the pixel electrode 18 made of a transparent conductive material such as indium tin oxide (ITO) are formed on the same plane.
The in-plane mode LCD with the 2ITO structure in which the pixel electrode and the common electrode are all formed as transparent electrodes, has advantages in that since the electrodes in the pixel region (an image display region) are formed as the transparent electrodes, an aperture ratio is increased, and since the two types of electrodes are formed on the same plane, the interval between the electrodes is uniform, thereby providing a good response speed and a residual image.
However, the in-plane mode LCD with the 2ITO structure has a problem in that since the electrodes are made of the transparent material, they do not exhibit complete black luminance in a normally black mode and thus a contrast ratio of a screen is not good.
In fabricating a liquid crystal display panel, the larger the panel becomes, the resistance of the electrodes increases which in turn affects the picture quality. In this respect, changing the structure of the existing design of the LCD cannot solve the resistance problem. In particular, the resistance of the transparent electrodes creates a big problem. If the electrodes are made thick in order to lower the resistance of the electrodes, the liquid crystal molecules manifest abnormal behavior due to a step according to the thickness of the electrodes, thereby degrading the picture quality, e.g., due to light leakage.